There are many applications for machine tools that require the motion of two points in space to be controlled relative to each other, in both position and angle, over a swept area or volume. It is desirable to minimise the number of machine axes involved to optimise this control. It is also desirable to maintain a very tight stiffness loop between the two points and ideally a constant value of stiffness in the loop as the position and angle of the points are adjusted. This improves the level of accuracy and repeatability of the motion.
Existing machine tools often use long linear guide rails to enable contact between a cutting tool (such as a grinding wheel) and a workpiece at any position along the workpiece's length. Shorter linear guide rails may be carried by the long rails in order to facilitate motion of a cutting tool towards or away from the workpiece, in a direction orthogonal to the long rails. These stacked rails (or axes) introduce unwanted compliance, reducing the tool-to-component stiffness. This in turn leads to reduced component quality, in terms of its dimensional precision and finish. Also, the long linear guide rails typically need to be least as long as the workpiece to be machined. This often results in an axis with a poor bearing ratio where the axis is most compliant in the direction of the cutting force. This problem is compounded when an orthogonal infeed axis is stacked upon the long axis.
The use of stacked axes is also problematic if on-axis position encoders are employed. The taller the stack of axes, the greater the distance between the points of interest and the encoders. This results in “Abbe offset” errors which reduce the intrinsic precision of the machine tools.
Furthermore, the use of orthogonal stacked linear axes requires time consuming and expensive alignment to maintain orthogonality between the axes and to minimise the pitch, yaw and roll errors for each axis.
These long linear axes also require long telescopic covers that are expensive, introduce friction, prone to failure and furthermore may influence the precision of linear motion (for example its straightness, positioning precision and repeatability).
The present invention seeks to overcome the problems above associated with use of long guide rails, and reduce the need for stacked, orthogonal axes.
International publication no. WO2009/093064 (in the name of the present applicant) describes a machine tool which comprises a machine base, a first support mounted on a first rotational machine axis on the base, and a second support mounted on a second rotational machine axis on the base. The second rotational axis is parallel to and spaced laterally from the first rotational axis and carries a mount moveable relative to the second support along a first linear machine axis orthogonal to the second rotational axis. A control arrangement is provided which is operable to control the orientation of the first support on the first rotational axis, and the orientation of the mount relative to the second rotational axis and its location along the linear axis, so as to govern the position and orientation of the first support and the mount relative to each other. This can be achieved without needing long linear axes and a stacked linear axis, thereby overcoming problems associated with the known configurations discussed above.